As a result of significant technological advances over the past decade, implantable medical devices (IMDs) can now be equipped with processing circuitry and memories for storing episodes of digitized physiological signals. For example, a pacemaker may be designed to perform the task of sensing, digitizing, and storing intracardiac signals for later uploading to an external device. The pacemaker need not be restricted to acquiring cardiac signals, but can also be used for obtaining, digitizing, and storing sensor signals, e.g., pressure sensor signals. Such signals are conventionally continuously sampled at a high rate in order to capture the highest anticipated or relevant frequency, such as those which occur during the actual contraction of the heart. For physiological heart signals, the importance is to be able to measure the amplitude of a signal during a contraction accurately. Typically, during cardiac contraction, the signals contain much more high frequency components compared to the situation between two contractions. This means that when the high frequencies are left out (which happens when the signal is sampled at a too low frequency), the amplitude is measured inaccurately. Examples of systems already in use which obtain and store digitized physiological signals are pacemakers, cardioverter defibrillator units and implantable hemodynamic monitors.
In IMDs, the task of digitizing data for storage and transmission to an external receiver becomes difficult because of the need to conserve data processing time and power. Generally available memory is a limitation, but even where the memory limitation is not significant, a data compression scheme is needed to save power during the data processing, telemetry communication, and the sampling process. It is known that data compression techniques can be used in order to increase the amount of signal information available for storage in available memory. See, for example, commonly assigned pending U.S. application Ser. No. 08/561,738, P-3432, “System and Method for Compressing Digitized Signals in Implantable and Battery-Powered Devices,” filed Nov. 22, 1995.
Because of the limited availability of processing resources and power, compression techniques for IMDs need to focus on achieving the greatest degree of data compression within the confines of allowable information loss. Thus, some lossy compression can be utilized in IMDs, to reach higher compression ratios with limited signal distortion.